1. Field of the Invention
The present technique relates to a technique employed in an imaging apparatus that captures continuous images, such as a digital camera, a video camera, or the like, and relates to a technique for removing images negatively influenced by incoming flash light.
2. Description of the Related Art
There exists, as a conventional signal processing apparatus (imaging apparatus) that removes images negatively influenced by incoming flash light, an apparatus as described, for example, in Patent Document 1 (JP 2007-306225A). FIG. 16 is a block diagram of this conventional imaging apparatus 900. Although the aforementioned Patent Document 1 describes processing for recording images into a recording unit, an example in which an image signal is outputted to the exterior shall be described here.
As shown in FIG. 16, the imaging apparatus 900 has a CMOS image sensor, a sensor driving circuit, and an analog/digital converter, and includes an imaging unit 950 that captures a continuous image and outputs a digital sequential scan image signal having, for example, 720 active vertical lines, 1280 active horizontal pixels, and a frame frequency of 60 Hz, as well as an image processing unit 951 that performs gamma processing (gamma correction), contour emphasis, and the like on the output from the imaging unit 950. The imaging apparatus 900 also includes a flashing light detection unit 952 that detects the presence/absence of incoming flash light by comparing the images in sequential frames in the output from the imaging unit 950, and an output control unit 953 that performs control so as to switch between outputting the image signal outputted by the image processing unit 951 directly or outputting images that have been accumulated, based on a signal outputted from the flashing light detection unit 952 (a flashing light detection signal).
FIG. 17 shows a block diagram of the output control unit 953 of the conventional imaging apparatus 900. The output control unit 953 is configured, for example, of the circuit shown in FIG. 17.
As shown in FIG. 17, the output control unit 953 includes a memory circuit 954 that stores one frame's worth of the image signal outputted from the image processing unit 951, and a switching circuit 955 that switches between and outputs the signal outputted from the memory circuit 954 or the signal inputted from the image processing unit 951 based on the signal outputted by the flashing light detection unit 952 (the flashing light detection signal).
Operations of the conventional imaging apparatus 900 configured as described above shall be described hereinafter.
First, in the imaging unit 950, the sensor driving circuit supplies a driving signal to the CMOS image sensor; an optical signal (light from a subject) that enters the CMOS image sensor is then converted to an electric signal through photoelectric conversion, and the electric signal obtained through the photoelectric conversion is then converted into a digital sequential scan image signal through analog/digital conversion. The digital sequential scan image signal generated by the imaging unit 950 is then outputted from the imaging unit 950. This digital sequential scan image signal is then supplied (outputted) to the image processing unit 951 and the flashing light detection unit 952.
The image processing unit 951 performs gamma processing, contour emphasis processing, and the like on the inputted digital sequential scan image signal, and then outputs the processed image signal to the output control unit 953.
Meanwhile, the flashing light detection unit 952 calculates the average luminance level of each frame for the inputted digital sequential scan image signal, and then compares the average luminance level of a frame of the inputted signal with the average luminance level of the frame immediately previous thereto. If the average luminance level has increased significantly, it is determined that the captured image was obtained by the imaging unit 950 in an environment in which an incoming flash light, such as a flash, was present. For example, if the average luminance level of a frame of the inputted signal has increased 100% or more compared to the average luminance level of the frame immediately previous thereto, the flashing light detection unit 952 determines that an incoming flash light such as a flash has been detected (that is, that the image was captured in environment in which an incoming flash light was present), sets the signal value of the flashing light detection signal to “1”, and outputs the flashing light detection signal. Note that when incoming flash light has not been detected, the flashing light detection unit 952 sets the signal value of the flashing light detection signal to “0” and outputs the flashing light detection signal. This flashing light detection signal and the image signal outputted from the image processing unit 951 are inputted into the output control unit 953.
In the output control unit 953, the image signal outputted from the image processing unit 951 is inputted into the memory circuit 954. The memory circuit 954 writes and stores one frame's worth of image signal, delays the signal by an amount of time equivalent to one frame, and outputs the image signal. In the case where the signal value of the flashing light detection signal outputted from the flashing light detection unit 952 is “1”, or in other words, in the case where flashing light has been detected, the memory circuit 954 does not write a new image signal, and instead holds the image signal of the previous frame and then repeats the output of the image signal of that same frame. The switching circuit 955 switches output between the image signal outputted from the image processing unit 951 and the image signal outputted from the memory circuit 954 based on the flashing light detection signal. In the case where the signal value of the flashing light detection signal outputted from the flashing light detection unit 952 is “1”, or in other words, in the case where the flashing light detection signal indicates that flashing light has been detected, the switching circuit 955 selects the image signal outputted from the memory circuit 954 and outputs that signal. However, in the case where the signal value of the flashing light detection signal outputted from the flashing light detection unit 952 is “0”, or in other words, in the case where the flashing light detection signal indicates that flashing light has not been detected, the switching circuit 955 selects the image signal outputted from the image processing unit 951 and outputs that signal.
Next, operations of the imaging apparatus 900 shall be described in more detail using FIG. 18.
FIG. 18 is a diagram illustrating negative influence on an image (an image captured by the imaging apparatus 900) caused by incoming flash light in the conventional imaging apparatus 900.
(1) to (4) in FIG. 18 indicate sequential frames, and the timing at which the CMOS image sensor is receiving light is indicated by a vertical synchronization signal (the uppermost section of FIG. 18).
The “upper side of the image” shown in FIG. 18 refers to the upper side of the active screen (the portion of the screen corresponding to the image that is actually displayed when an image signal obtained by the imaging apparatus is displayed in a display apparatus (that is, the image excluding portions such as the synchronization signals)). In the CMOS image sensor, the first line of the active screen receives light during the period indicated by the arrow in FIG. 18. Because the timing at which light is received shifts slightly with each line in the CMOS image sensor, the 720th line is shifted by approximately 1/60 of a second relative to the first line. Here, in the case where incoming flash light has occurred at the timing indicated by the star in FIG. 18, that flashing light will be received in the lines that compose the lower side of the screen in the frame (2) and the lines that compose the upper side of the screen in the frame (3). When, in such a state, the image signal obtained by the imaging unit 950 is displayed in a display apparatus, a horizontal band of high luminance will appear across the two sequential frames in the displayed screen (video). The appearance of such a horizontal band of high luminance will impart a sense of visual unnaturalness in a user. Furthermore, in methods that perform compression using the correlation between continuous frames, such as, for example, processing that compresses according to the MPEG (Moving Picture Experts Group) method, processing an image signal in which the aforementioned horizontal band of high luminance has appeared causes negative influence, such as the occurrence of block noise, a decrease in compression rates, and so on.
Accordingly, with the conventional imaging apparatus 900 shown in FIG. 16, an image in a frame in which flashing light has been received is not outputted when incoming flash light has been detected; rather, the image of the frame immediately previous thereto is outputted. Therefore, the imaging apparatus 900 can prevent the appearance of horizontal bands of high luminance.